A mechanism of the type with which the invention is concerned is described in U.S. Pat. No. 4,559,972. A sealing plate is connected by a linkage to the rotary plug, which closes the inlet opening. When that mechanism is in its closed position, the sealing plate will always assume the same position on the valve seat, which is associated with the outlet opening. This may result in the accumulation of contaminants or a high rate of wear leading to loss of sealing or to maintenance problems.